1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a protected transition-metal hexacyanoferrate (TMHCF) cathode battery and associated fabrication processes.
2. Description of the Related Art
A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. The energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential of −3.04 V vs. H2/H+, and has the highest gravimetric capacity of 3860 milliamp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries have led the portable electronics revolution. However, the high cost of lithium metal renders doubtful the commercialization of lithium batteries as large scale energy storage devices. Further, the demand for lithium and its reserve as a mineral have raised the need to build other types metal-ion batteries as an alternative.
Lithium-ion (Li-ion) batteries employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. As a battery is cycled, lithium ions (Li+) are exchanged between the positive and negative electrodes. Li-ion batteries have been referred to as rocking chair batteries because the lithium ions “rock” back and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material having a tunneled structure, such as lithium manganese oxide (LiMn2O4), on an aluminum current collector. The negative electrode (anode) material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge-discharge process, lithium ions are inserted into, or extracted from interstitial spaces of the active materials.
Similar to the lithium-ion batteries, metal-ion batteries use the metal-ion host compounds as their electrode materials in which metal-ions can move easily and reversibly. As for a Li+-ion, it has one of the smallest radii of all metal ions and is compatible with the interstitial spaces of many materials, such as the layered LiCoO2, olivine-structured LiFePO4, spinel-structured LiMn2O4, and so on. Other metal ions, such as Na+, K+, Mg2+, Al3+, Zn2+, etc., with large sizes, severely distort Li-based intercalation compounds and ruin their structures in several charge/discharge cycles. Therefore, new materials with large interstitial spaces would have to be used to host such metal-ions in a metal-ion battery.
FIG. 1 is a diagram depicting the crystal structure of a transition-metal hexacyanoferrate (TMHCF) in the form of AxM1M2(CN)6 (prior art). TMHCFs with large interstitial spaces have been investigated as cathode materials for rechargeable lithium-ion batteries [1,2], sodium-ion batteries [3,4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates ((Cu,Ni)-HCFs) exhibited a very good cycling life that 83% capacity was retained after 40,000 cycles at a charge/discharge current of 17C [6-8]. However, the materials demonstrated low capacities and energy densities because: (1) just one sodium-ion can be inserted/extracted into/from each Cu—HCF or Ni—HCF molecule, and (2) these TMHCFs electrodes must be operated below 1.23 V due to the water electrochemical window. To correct these shortcomings, manganese hexacyanoferrate (Mn—HCF) and iron hexacyanoferrate (Fe—HCF) were used as cathode materials in non-aqueous electrolyte [9, 10]. Assembled with a sodium-metal anode, the Mn—HCF and Fe—HCF electrodes cycled between 2.0V and 4.2 V and del red capacities of about 110 mAh/g.
However, some TMHCFs electrodes with high capacities exhibited rapid capacity degradation with cycling, when they were used in rechargeable batteries with non-aqueous electrolyte. For example, the capacity of Fe-HCF electrodes in lithium-ion batteries decreased from 110 mAh/g to 80 mAh/g after 10 cycles [2]. A Cu-HCF electrode with Li-ion electrolyte delivered 120 mAh/g during the first discharge, but its capacity decreased to 40 mAh/g after 10 cycles [11]. On one hand, the capacity fading can be attributed to many factors related to the TMHCFs themselves, such as their phase changes during charge/discharge, interstitial water, and metal-ions dissolution from TMHCFs to the electrolyte. On the other hand, the interaction between TMHCFs electrodes and electrolytes also affect their performance. The side reactions of electrolytes on the surfaces of Cu-FICFs electrodes induced the redox-inactive Fe(II) ions and affected their electronic structures [12]. To improve the cycling life of Cu—HCFs, a core (Cu—HCF)-shell (Ni—HCF) electrode materials was prepared and evaluated in Li-ion batteries [12]. After 50 cycles the core-shell electrode had about 65% capacity left, which was much better than Cu—HCF electrode (about 20% capacity retention).
It is worth noting that the actual capacities of TMHCFs electrodes are far smaller than their theoretical values. For instance, the theoretical capacity for Mn-HCF is 170 mAh/g, but the capacity was just reported ˜120 mAh/g, as it was tested in a sodium-ion battery. It was believed that structures of TMHCFs determined their performance. Buser et al. [13] investigated the crystal structure of Prussian blue (PB) Fe4[Fe(CN)6]3·mH2O and found that the Fe(CN)6, positions were only partly occupied. The vacancies led to much water entering the PB interstitial space and even associating with Fe(III) in the lattice [14]. In consideration of charge neutralization and interstitial space, the vacancies and water reduced the concentration of mobile ions in the interstitial space of TMHCFs. As an example, Matsuda, et al. [9] preferred to use A4x−2MA[MB(CN)6]x·zH2O to replace the nominal formula, A2MAMB(CN)6 because of the vacancies. Furthermore, the vacancies result in dense defects on the surface of TMHCFs. Without interstitial ions and water support, the surface easily collapses. The surface degradation can be aggravated when the interstitial ions near the surface are extracted out during electrochemical reactions. In a battery, this degradation leads to a poor capacity retention. As noted above, a Cu—HCF electrode with Li-ion electrolyte delivered 120 mAh/g during the first discharge, but its capacity decreased to 40 mAh/g after 10 cycles [11]. By coating with Ni—HCF, the surface of Cu—ICF was modified and its stability was improved. However, undercoordinated transition metals (UTM) on the surface retarded charge transfer between the TMHCF electrode and the electrolyte due to electric repulsion between UTM and mobile ions, resulting in poor electrochemical performance. Park et al. [15] noted that the surface effect on a LiFePO4 electrode, with undercoordinated Fe2+/Fe3+ at the surface, created a barrier for Li+ transport across the electrolyte/electrode interface.                [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.        [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.        [3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48(2012)6544-6546.        [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J. B. Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52(2013)1964-1967.        [5] A. Eftekhari., Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.        [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2(2011) 550.        [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Letters, 11(2011) 5421-5425.        [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159(2012) A98-A103.        [9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.        [10] S. H. Yu, M. Shokouhimehr, T. Hyeon, Y.-E. Sung, iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2(2013)A39-A41.        [11] D. Asakura, C, H. Li, Y. Mizuno, M. Okubo, H. Zhou., D. R. Talhani, Bimetallic cyanide-bridged coordination polymers as lithium ion cathode materials: core@shell nanoparticles with enhanced Cyclability, J. Am. Chem. Soc., 135(2013)2793-2799.        [12] M. Okubo, D. Asakura, Y. Mizuno, T. Kudo, H. Zhou, A. Okazawa, N. Kojima, K. Ikedo, T. Mizokawa, I. Honma, Ion-induced transformation of magnetism in a bimetallic CuFe Prussian blue analogue, Angew, Chem, Int. Ed., 50(2011)6269-6273.        [13] H. J. Buser, D. Schwarzenbach, W. Petter, A. Ludi, the crystal structure of Prussian blue: Fe4[Fe(CN)6]3.xH=O, Inorganic Chemistry, 16(1977) 2704-2710.        [14] F. Herren, P. Fischer, A. Ludi, W. Hälg, Neutron diffraction study of Prussian blue, Fe4[Fe(CN)6]3.xH2O. Location of water molecules and long-range magnetic order, Inorg. Chem. 1980, 19, 956-959        [15] K.-S. Park, P. Xiao, S.-Y. Kim, A. Dylla, Y.-M. Choi, G. Henkelman, K. J. Stevenson, J. B. Goodenough, Enhanced charge-transfer kinetics by anion surface modification of LiFePO4, Chem. Mater. 24(2012)3212-3218.        
It would be advantageous if a TMHCF cathode could be treated or modified in such a manner as to support the lattice structure through multiple cycles of charge and discharge.